The present inventions relate to solar-thermal receiver systems, and, more particularly, to molten salt receiver systems having a cooling system to provide cooling upon a predetermined flow condition to permit redirecting heliostats from the receiver heat exchanger.
Molten salt receiver systems are used in solar-thermal energy conversion systems. One of the most common applications of the molten salt system applies to solar tower systems such as is described in U.S. Pat. No. 5,417,052 to Bharathan et al. A similar system is generally depicted in FIG. 1 illustrating an array or a field of heliostats 112 that follow the sun 110 and re-direct the solar radiation 114 to a central receiver system located atop a tower 118.
The central receiver system typically has a receiver heat exchanger 116 that becomes heated by the reflected solar energy 114. The heat is transferred to a molten salt, typically nitrate salts such as a combination of liquid sodium nitrate and potassium nitrate. The heated molten salt is directed to a hot storage tank 122 that provides energy to a power generation system 124. The molten salt is returned to a cold storage tank 120 that supplies the central receiver system. In the power generation system 124, the molten salt exchanges heat through a heat exchanger to a fluid such as water or air. For example, as described in the Bharathan patent, the molten salt is directed to an air heater that drives a hybrid gas power plant. A gas turbine converts the thermal energy into mechanical energy, which is then converted into electrical energy by a generator.
In molten salt receiver systems, the molten salt is not only a thermal storage fluid but also functions as a coolant that reduces the temperature of the receiver heat exchanger 116. If the flow of molten salt is inadvertently reduced or terminated, the solar energy will quickly overheat the receiver heat exchanger 116. In such a situation, it is desirable to defocus the heliostats 112 by moving them to a standby position so that solar energy is not directed toward the receiver heat exchanger 116. However, an adequate supply of molten salt must be available to cool the receiver heat exchanger 116 for the amount of time required to redirect the heliostats 112.
One molten salt receiver system 200 was devised to cause cooling while redirecting heliostats and is rather simply described by FIG. 2. In this system, molten salt storage tanks 220, 222 were located at ground level. A cold storage tank 220 stored the supply of cold molten salt to be sent to the receiver heat exchanger 216, and a hot storage tank 222 stored hot molten salt for use in an energy generation system. A variable speed pump 224 provided molten salt from the cold storage tank 220 to a pressurized receiver inlet vessel 226, which physically sat level to or slightly below the receiver heat exchanger 216. The receiver inlet vessel 226 supplied flow of molten salt to the receiver heat exchanger 216 by way of a controlled throttle valve 228. The throttle valve 228 controlled flow through the heat exchanger 216 based upon optimum heat exchange conditions. Typically, flow is throttled to maintain a receiver heat exchanger 216 outlet temperature of about 1050xc2x0 F. After flowing through the receiver heat exchanger 216, molten salt entered a receiver outlet vessel 230, which sat physically slightly higher than the receiver heat exchanger 216. The outlet vessel 230 was vented to atmosphere and permitted the release of entrained air in the molten salt. From the receiver outlet vessel 230, a downcomer 231 provided the flow of molten salt to a hot storage tank 222.
In this molten salt receiver system 200, the downcomer 231 required a full head of molten salt in order to avoid the effects of potentially destructive rapid momentum changes to the flow. The receiver outlet vessel 230 included a level indicator (not shown) assuring that the downcomer 231 was full. Feedback controlled active drag valves 232, 233 were located at the base of the downcomer 231 and maintained the head. At least two active drag valves 232, 233 were provided for redundancy. As such, the drag valves 232, 233 were controlled by feedback from the level indicator at the receiver outlet vessel 230.
The pressurized receiver inlet vessel 226 served an emergency cooling purpose in the system 200. The receiver inlet vessel 226 provided a flow of molten salt for cooling the receiver heat exchanger 216 in the event of loss of normal flow. When flow ceases, the heliostats are required to be redirected from the receiver heat exchanger 216, a process that typically takes about one minute. In order to avoid overheating the receiver heat exchanger 216, the flow from the receiver inlet vessel 226 must be provided during this one minute. Therefore, the level of the inlet vessel 226 was maintained at a predetermined value to keep at least a minute""s worth of supplemental molten salt flow.
Because the drag valves 232, 233 were actively opened by a control system, they could inadvertently close. In the absence of free space in the outlet vessel 230, the inadvertent shutting of the drag valves 232, 233 could prevent flow of the molten salt through the receiver heat exchanger 216. As such, the level in the receiver outlet vessel 230 required available free space to accept the required amount of flow from the receiver inlet vessel 226 during the one minute period required to redirect the heliostats.
From the foregoing description, it becomes apparent that there are several control variables involved in the molten salt receiver system 200. First, the flow through the heat exchanger 216 was controlled by the throttle valve 228 according to optimum heat exchange requirements, which vary greatly according to the intensity of sunlight throughout the day. In fact, cloud cover transients can have a sudden and dramatic effect on the temperature and necessary flow through the receiver heat exchanger 216. Secondly, the level of the receiver inlet vessel 226 was controlled to maintain an appropriate level as required to cool the receiver heat exchanger 216 in the event of loss of flow. A variable speed pump 224 supplying the receiver inlet vessel 226 maintained appropriate pressure in the vessel 226. Also, the variable speed pump 224 and an air feed and bleed cover gas system (not shown) maintained the appropriate level in the inlet vessel 226. As can be seen, controlling the receiver inlet vessel level was also dependent upon the throttled flow through the receiver heat exchanger 216, therefore affecting control of the speed of the pump 224. Thirdly, the level of the receiver outlet vessel 230 was required to be maintained in a band, high enough to indicate a full head in the downcomer 231, yet low enough to permit a minute""s worth of volume to flow from the receiver inlet vessel 226 in the event of inadvertent drag valve 232, 233 closure. The drag valve 232, 233 controlled the level of the receiver outlet vessel 230 by feedback from the level indicator (not shown). As can be seen, the level of the receiver outlet vessel 230 is also dependent upon the flow through the heat exchanger 216.
The cross dependence of the control variables in the foregoing system created a rather elaborate and complex control system for the molten salt receiver system 200. As such, a need in the art exists for a less complex control system. However, the system must maintain the ability to provide adequate molten salt cooling to the receiver heat exchanger 216 in the event of a loss of flow. Additionally, the system must be capable of preventing momentum changes of the downcomer flow that can induce undesirable mechanical forces in the downcomer.
Therefore according to the present inventions, a molten salt receiver system and method for cooling a receiver heat exchanger of a molten salt receiver are provided. According to one embodiment of the present invention, a system for cooling a receiver heat exchanger of a molten salt receiver system includes a molten salt holding vessel upstream of the receiver heat exchanger. The holding vessel contains a store of molten salt that is maintained at an appropriate temperature to cool the receiver heat exchanger. An air separator downstream of the receiver heat exchanger permits the release of air entrained in the molten salt flow from the receiver heat exchanger. A downcomer downstream of the air separator delivers molten salt to a molten salt energy generation system, which according to one aspect of the invention includes a storage tank. According to one aspect of the invention, the downcomer includes at least one flow obstacle. The flow obstacle, such as a passive flow restrictor or a turbine, permits controlled flow through the downcomer avoiding rapid flow and momentum changes in the molten salt. These momentum changes often create sudden and violent physical vibration of the downcomer and associated system. Passive flow restrictors include any of many commercially available flow restricting devices such as orifices, baffles, or open cell metal foam.
According to one aspect of a molten salt receiver system of the present invention, at least one isolation valve connects the holding vessel and the receiver heat exchanger. The isolation valve can open upon a predetermined flow condition in the system, which requires cooling of the receiver heat exchanger. Typically, these are low flow or loss of flow conditions. Such conditions are typically monitored by flow sensors or pump power supplies. A predetermined threshold may be established for each of these conditions, below which the isolation valve automatically opens. According to one aspect of the invention, the holding vessel has a capacity to supply a volume of molten salt corresponding to a volume required to cool the receiver heat exchanger for at least one minute. In this example, one minute is chosen as a minimum so that adequate time is provided to move heliostats that are providing thermal energy to the receiver heat exchanger. Of course, other embodiments may require more or less cooling time depending upon heat exchange characteristics of the receiver heat exchanger and the time required to redirect or defocus heliostats.
According to another aspect of the instant molten salt receiver system, a pressurization system is connected to the holding vessel and adapted to permit selective pressurization of the holding vessel. Other aspects of the pressurization system permit selective venting of the vessel, either by an overpressure relief or a controlled venting valve. As thus far described, both the holding vessel and the air separator are vented, and as such it is advantageous to include a vent system in the molten salt receiver system. Therefore, according to one aspect of the molten salt receiver system, the vent system vents the air separator and the holding vessel to a molten salt storage tank.
The present invention also includes other elements of a molten salt receiver system including a molten salt source. This tank is typically a cold tank for storing molten salt kept in fluid form. A molten salt pump receives molten salt from the molten salt source providing flow through the receiver heat exchanger. The outlet of the pump is interconnected to the outlet of the holding vessel. As such, the previously described isolation valve isolates the holding vessel from the flow of the pump through the receiver heat exchanger, that is until the isolation valve opens. Upon opening, the flow from the pump is supplemented by the holding vessel. Additionally, a flow control valve is downstream of the pump and controls the normal flow through the receiver heat exchanger.
Another embodiment of the present invention includes a method for cooling a receiver heat exchanger of a molten salt receiver system. The method comprises permitting flow of molten salt through the receiver heat exchanger. After the molten salt is passed through the heat exchanger, air entrained in the flow is released. The flow is then delivered to a molten salt energy generation system, such as a hot storage tank. The delivery of molten salt to the hot storage tank also includes passively restricting flow of molten salt. Passive restriction, as previously described, avoids undesirable effects of sudden fluid flow momentum changes.
The flow is also monitored to detect an occurrence of a predetermined flow condition, such as monitoring flow through the receiver heat exchanger. The predetermined flow condition may correspond to flow falling below a predefined threshold. Alternatively, the monitoring includes monitoring an electric pump power supply to detect a predetermined flow condition corresponding to loss of power to the pump. Upon the detection of the occurrence of the predetermined flow condition, the flow is automatically supplemented from a molten salt holding vessel. According to one aspect, the supplementation of flow is initiated by automatically opening a valve isolating the molten salt holding vessel. As described above, it is desirable to supplement flow for at least one minute. Therefore, the molten salt holding vessel is filled to a level corresponding to a volume required during the step of automatically supplementing flow.